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United States Patent |
6,192,573
|
Hahakura
,   et al.
|
February 27, 2001
|
Method of preparing oxide superconducting wire
Abstract
An oxide superconducting wire having a circular or substantially circular
sectional shape and exhibiting a high critical current density comparable
to that of a tape-shaped wire is provided. The oxide superconducting wire
consists of a plurality of filaments extending along the longitudinal
direction of the wire in the form of ribbons, and a stabilizer matrix
covering the filaments. The aspect ratio of the width to the thickness of
each filament is 4 to 40, and the thickness of each filament is 5 to 50
.mu.m. A section of the wire is in a circular or substantially circular
shape. The wire exhibits a critical current density of at least 2000
A/cm.sup.2 at a temperature of 77 K with no application of a magnetic
field. It is preferable that the plurality of filaments are substantially
rotation-symmetrically arranged with respect to the center of the wire. It
is also preferable that a hexagonal-prismatic stabilizing matrix is
provided at the center of the wire and the plurality of filaments covered
with the stabilizer matrix are arranged on each side surface thereof in a
layered manner. A flat stranded wire having low ac loss can be formed by
such wires.
Inventors:
|
Hahakura; Shuji (Osaka, JP);
Saga; Nobuhiro (Osaka, JP);
Ohmatsu; Kazuya (Osaka, JP);
Sato; Kenichi (Osaka, JP)
|
Assignee:
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Sumitomo Electric Industries, Ltd. (Osaka, JP)
|
Appl. No.:
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055287 |
Filed:
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April 6, 1998 |
Foreign Application Priority Data
Current U.S. Class: |
29/599; 174/125.1; 505/231; 505/704 |
Intern'l Class: |
H01L 039/24 |
Field of Search: |
29/599
505/431,231,704
174/125.1
|
References Cited
U.S. Patent Documents
4414428 | Nov., 1983 | McDonald | 29/599.
|
4501062 | Feb., 1985 | Hillmann et al. | 29/599.
|
4652697 | Mar., 1987 | Ando et al. | 29/599.
|
4885273 | Dec., 1989 | Sugimoto et al. | 29/599.
|
4952554 | Aug., 1990 | Jin et al. | 29/599.
|
5068219 | Nov., 1991 | Hagino et al. | 29/599.
|
5232908 | Aug., 1993 | Shiga et al. | 29/599.
|
5283232 | Feb., 1994 | Kohno et al. | 29/599.
|
5340943 | Aug., 1994 | Sato | 29/599.
|
5885938 | Mar., 1999 | Otto et al. | 29/599.
|
Foreign Patent Documents |
0451864A2 | Oct., 1991 | EP.
| |
0503525A1 | Mar., 1992 | EP.
| |
0134822 | May., 1989 | JP | 29/599.
|
4-262308 | Sep., 1992 | JP.
| |
4-329218 | Nov., 1992 | JP.
| |
5-101722 | Apr., 1993 | JP | 505/431.
|
5-250937 | Sep., 1993 | JP | 505/431.
|
6-44832 | Feb., 1994 | JP | 505/431.
|
6-44833 | Feb., 1994 | JP | 505/431.
|
6-68729 | Mar., 1994 | JP | 505/431.
|
6-208809 | Jul., 1994 | JP | 505/431.
|
Other References
C.H. Rosner et al., "Status of HTS Superconductors: Progress in Improving
Transport Critical Current Densities in HTS Bi-2223 Tapes and Coils",
Cryogenics, vol. 32, No. 11, (1992), pp. 940-948.
|
Primary Examiner: Arbes; Carl J.
Attorney, Agent or Firm: Foley & Lardner
Parent Case Text
This application is a divisional of application Ser. No. 08/823,907, filed
Mar. 25, 1997.
Claims
What is claimed is:
1. A method of preparing an oxide superconducting wire comprising a
plurality of oxide superconductor filaments embedded in a stabilizer, said
method comprising:
charging powder of a member selected from the group consisting of said
oxide superconductor and a raw material for said oxide superconductor in a
tube or tubes consisting essentially of said stabilizer;
drawing and rolling said tube or tubes charged with said powder to obtain a
plurality of tape-shaped wires, each having a portion of said powder in
the form of a ribbon having an aspect ratio of 4 to 40;
arranging said plurality of tape-shaped wires in parallel on a sheet
consisting essentially of a stabilizer;
winding said sheet with said tape-shaped wires on a substantially circular
stabilizer;
inserting said circular stabilizer on which said tape-shaped wires are
wound into a cylindrical tube consisting essentially of a stabilizer;
performing plastic working on said tube in which said tape-shaped wires
have been inserted to obtain a wire having a shape selected from the group
consisting of a substantially circular sectional shape and an at least
hexagonal polygonal sectional shape which is substantially
rotation-symmetrical; and
performing a heat treatment on said wire to obtain a sintered oxide
superconductor and produce an oxide superconducting wire comprising a
plurality of oxide superconductor filaments embedded in a stabilizer, each
oxide superconductor filament having a thickness within the range of 5 to
50 .mu.m.
2. The method of preparing an oxide superconducting wire in accordance with
claim 1, wherein said plurality of tape-shaped wires are inserted in said
tube in a charge density of at least 90% in said step of inserting said
plurality of tape-shaped wires.
3. The method of preparing an oxide superconducting wire in accordance with
claim 1, wherein said step of performing plastic working on said tube
charged with said tape-shaped wires comprises drawing with a driving
roller die.
4. The method of preparing an oxide superconducting wire in accordance with
claim 1, further comprising the steps of performing drawing for attaining
an area reduction ratio of at least 5% and not more than 50% on a section
of said wire and then performing a heat treatment on said wire for
sintering said oxide superconductor.
5. The method of preparing an oxide superconducting wire in accordance with
claim 4, wherein said drawing is performed with a driving roller die.
6. The method of preparing an oxide superconducting wire in accordance with
claim 1, wherein said stabilizer is selected from the group consisting of
silver, silver alloys and combinations thereof, said heat treatment is
performed at a temperature within the range of 700 to 900.degree. C., and
said oxide superconductor is a bismuth oxide superconductor mainly
composed of a bismuth 2223 or 2212 phase.
7. The method in accordance with claim 1, wherein, following said heat
treatment, said oxide superconducting wire is drawn to reduce the area by
at least 5% and not more than 50% on a section of said tube, and then
subjected to a further heat treatment.
8. The method in accordance with claim 7, wherein said drawing is performed
with a driving roller die.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a wire employing an oxide superconductor
and a method of preparing the same, and a stranded wire and a conductor
employing such wires, and more particularly, it relates to a wire having a
substantially circular sectional shape or a polygonal sectional shape
which is substantially rotation-symmetrical and exhibiting a high critical
current density, and structures of a stranded wire and a conductor having
small ac loss employing the wire.
2. Description of the Background Art
Among conventional oxide superconducting silver sheath wires, a round wire
having a circular section exhibits an extremely small critical current
density as compared with a tape-shaped wire obtained through rolling. This
is conceivably because the density of a superconductor is not increased in
the wire due to absence of a rolling step in the process of preparing the
round wire. This is also conceivably because superconducting crystals are
not oriented by a lamellar structure which is specific to a bismuth
superconductor due to the absence of the rolling step and hence c-axis
orientation unsatisfactorily results from crystal growth in sintering.
As a result of development of long wires having critical current densities
exceeding 10.sup.4 A/cm.sup.2, application of silver sheath bismuth oxide
superconducting wires to superconducting power apparatuses with liquid
nitrogen cooling is expected. Among such silver sheath bismuth oxide
superconducting wires, however, only the so-called tape-shaped wire
satisfies a practical critical current density, a long wire shape, a mass
production technique and the like under the present circumstances. The
tape-shaped wire is prepared by charging powder serving as raw material
for a bismuth oxide superconductor in a silver pipe, drawing the same,
engaging a plurality of such drawn silver pipes in a silver pipe for
obtaining a multifilamentary wire, further drawing the wire, and
thereafter rolling and heat treating the same.
Although a high capacitance conductor is experimentally prepared by
spirally winding the tape-shaped wire on a cylindrical pipe for attaining
a multilayer structure, high ac loss may be generated from the structure.
In ac application of a superconducting wire, ac loss resulting from a
fluctuating magnetic field comes into question. In a conductor prepared by
assembling superconducting wires, on the other hand, a problem of current
drift arises from ununiform impedances between the wires. Due to such
drift, ac loss generated in the conductor disadvantageously exceeds the
sum of those generated in the respective wires forming the conductor. This
problem takes place in a conductor obtained by superposing tape-shaped
wires in a layered manner.
In a conventional conductor employing metal superconducting wires,
impedances of filaments or the wires are uniformalized by twisting the
filaments or the wires or through transposition of the filaments. Also in
such a conductor employing oxide superconducting wires, it is important to
employ round wires in which current drift is prevented by transposition of
the superconductor itself, in order to reduce ac loss. In order to obtain
a high critical current density in the oxide superconductor of ceramics,
however, it has been necessary to reinforce grain bonding and increase
crystal orientation by shaping the wire into a tape and sintering the
same.
The oxide superconductor has a characteristic that the c-axis is oriented
on an interface between the same and a metal, and its critical current
density is improved with improvement of the c-axis orientation. The
tape-shaped oxide superconducting wire has a high critical current density
since its c-axis orientation is improved by pressing or rolling in working
into a tape shape, and further the density of a superconducting crystal
portion is increased. In case of preparing a round wire, however, only an
extremely low critical current density has been attained as compared with
the tape-shaped wire due to absence of the pressing or rolling step.
Japanese Patent Laying-Open No. 4-262308 (1992) discloses a round oxide
superconducting wire having a section in which a metal, silver or a silver
alloy and an oxide superconductor are alternately concentrically stacked
with each other, for improving the critical current density. This gazette
describes that c-axis orientation can be attained in this wire by
alternately stacking the metal and the oxide superconductor for attaining
a multiring structure and reducing the distance between the interfaces
between the oxide superconductor and the metal, specifically to not more
than 100 .mu.m between adjacent interfaces. Although this wire exhibits a
critical current density which is higher by one order as compared with
other conventional round wires, however, this critical current density
value is smaller by one order as compared with the tape-shaped wire, i.e.,
by far lower than a practically required critical current density level.
Another example aiming at improving the critical current density of the
round superconducting wire is disclosed in Cryogenics (1992) Vol. 32, No.
11, pp. 940 to 948. In the round wire described in the literature, 55
single-filamentary rods having rectangular sections are concentrically
arranged in a silver tube in three layers. No critical current is decided
as to the obtained wire in the literature. However, it is presumable that
the critical current density of the wire described in the literature is
not much high, as described later.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an oxide superconducting
wire having a circular sectional shape or a sectional shape similar
thereto and exhibiting a high critical current density comparable to that
of the tape-shaped wire.
Another object of the present invention is to provide a stranded wire or a
conductor employing a plurality of oxide superconducting wires, which can
further reduce ac loss.
The oxide superconducting wire according to the present invention is
prepared by a powder-in-tube method, and comprises a plurality of
filaments consisting essentially of an oxide superconductor and extending
along the longitudinal direction of the wire in the form of ribbons, and a
matrix consisting essentially of a stabilizer covering the plurality of
filaments. In this wire, the aspect ratio of each ribbon-shaped filament
is within the range of 4 to 40, and its thickness is within the range of 5
to 50 .mu.m. The wire has a substantially circular sectional shape or an
at least hexagonal polygonal sectional shape which is substantially
rotation-symmetrical, and exhibits a critical current density of at least
2000 A/cm.sup.2 with no application of a magnetic field at a temperature
of 77 K.
The wire according to the present invention can be provided with a
prismatic stabilizing matrix having an at least hexagonal substantially
regular-polygonal sectional shape substantially at its center, and the
filaments covered with the stabilizer are stacked on each side surface of
the prismatic stabilizing matrix in one or more layers. The prismatic
matrix can have a substantially regular-hexagonal sectional shape, for
example.
The wire according to the present invention can be provided with a
substantially cylindrical stabilizing matrix having a substantially
circular sectional shape substantially at its center, and the plurality of
filaments covered with the stabilizer can be spirally arranged around the
substantially cylindrical stabilizing matrix.
In the wire according to the present invention, the oxide superconductor is
preferably prepared from a bismuth based oxide superconductor mainly
composed of a bismuth 2223 or 2212 phase, while the stabilizer is
preferably prepared from any material selected from the group consisting
of silver, silver alloys and combinations thereof.
A method according to the present invention is adapted to prepare an oxide
superconducting wire comprising a plurality of filaments consisting
essentially of an oxide superconductor covered with a stabilizer, and
comprises the steps of charging powder of an oxide superconductor or raw
material therefor in a tube consisting essentially of a stabilizer,
performing plastic working on the tube charged with the powder to obtain a
tape-shaped wire, charging a plurality of such tape-shaped wires in a tube
consisting essentially of a stabilizer, performing plastic working on the
tube charged with the tape-shaped wires to obtain a wire having a
substantially circular sectional shape or an at least hexagonal polygonal
sectional shape which is substantially rotation-symmetrical, and heat
treating the wire for forming a sintered body of the oxide superconductor.
In each tape-shaped wire charged in the tube, a portion consisting of the
powder is in the form of a ribbon having an aspect ratio of 4 to 40.
In the method according to the present invention, the wire having a
substantially circular sectional shape or a polygonal sectional shape
which is substantially rotation-symmetrical is heat treated to obtain an
oxide superconducting wire comprising the filaments consisting essentially
of an oxide superconductor and each having a thickness within the range of
5 to 50 .mu.m.
In the method according to the present invention, the step of charging the
plurality of tape-shaped wires in the tube consisting essentially of a
stabilizer can comprise the steps of arranging a prismatic stabilizer
having an at least hexagonal substantially regular-polygonal sectional
shape substantially at the center of the tube and stacking the tape-shaped
wires on each side surface of the prismatic stabilizer in one or more
layers.
In the method according to the present invention, the step of charging the
plurality of tape-shaped wires in the tube consisting essentially of a
stabilizer can comprise the steps of providing a substantially cylindrical
stabilizer having a substantially circular sectional shape, arranging the
plurality of tape-shaped wires on a sheet consisting essentially of a
stabilizer in parallel with each other, winding the sheet provided with
the plurality of tape-shaped wires on the substantially cylindrical
stabilizer, and inserting the same in the tube.
In the method according to the present invention, the plurality of
tape-shaped wires can be charged in the tube in a charge density of at
least 90%, for example, in the step of charging the plurality of
tape-shaped wires in the tube consisting essentially of a stabilizer.
In the method according to the present invention, the step of performing
plastic working on the tube charged with the tape-shaped wires preferably
comprises drawing with a driving roller die.
In the method according to the present invention, the aforementioned heat
treatment may be followed by the steps of performing drawing for attaining
an area reduction ratio of at least 5% and not more than 50% in the
section of the wire and then heat treating the wire for sintering the
oxide superconductor. The drawing for attaining the area reduction ratio
of at least 5% and not more than 50% can be performed with a driving
roller die.
In the method according to the present invention, the stabilizer is
preferably selected from the group consisting of silver, silver alloys and
combinations thereof, and the heat treatment is preferably performed at a
temperature in the range of 700 to 900.degree. C., while filaments
consisting essentially of a bismuth based oxide superconductor mainly
composed of a bismuth 2223 or 2212 phase are preferably formed.
On the other hand, a superconducting stranded wire employing the inventive
oxide superconducting wire is provided. The superconducting stranded wire
is characterized in that a plurality of oxide superconducting wires
according to the present invention are twined and the stranded wire is
flatly shaped.
In the superconducting stranded wire according to the present invention, a
structure having the oxide superconducting wire twisted can be provided.
In the superconducting stranded wire according to the present invention, a
high-resistance metal layer or an insulating layer can be formed around
the oxide superconducting wires.
Further, a stranded wire can be provided by winding the oxide
superconducting wire or wires comprising filaments consisting essentially
of an oxide superconductor and a stabilizer covering the same, on the
aforementioned superconducting stranded wire.
In addition, a superconducting conductor employing the superconducting
stranded wires is provided according to the present invention. The
superconducting conductor is characterized in that the superconducting
stranded wire is spirally wound on a cylindrical or spiral core in one or
more layers.
The foregoing and other objects, features, aspects and advantages of the
present invention will become more apparent from the following detailed
description of the present invention when taken in conjunction with the
accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1(a) and 1(b) are perspective views schematically showing two
exemplary structures of the wires according to the present invention
respectively;
FIG. 2 is a perspective view schematically showing still another exemplary
structure of the wire according to the present invention;
FIG. 3 is a perspective view schematically showing a further exemplary
structure of the wire according to the present invention;
FIG. 4 is a side elevational view showing a driving roller die employed in
the present invention;
FIG. 5 is a perspective view schematically showing an exemplary structure
of the stranded wire according to the present invention;
FIG. 6 is a sectional view showing a pipe charged with flat wires for
engagement along with a hexagonal-prismatic stabilizer in Example 1 of the
present invention;
FIG. 7 is a sectional view showing a pipe charged with flat wires for
engagement along with a cylindrical stabilizer in Example 1 of the present
invention;
FIG. 8 is a sectional view showing a pipe charged with flat wires for
engagement as many as possible in Example 1 of the present invention;
FIG. 9 is a side elevational view showing a polygonal driving roller die
employed in the present invention;
FIG. 10 is a sectional view showing a wire drawn with the polygonal driving
roller die in Example 1 of the present invention;
FIG. 11 is a sectional view schematically showing a flat stranded wire
obtained in Example 4 of the present invention;
FIG. 12 is a schematic sectional view showing a wire obtained by stacking
12 tape-shaped wires in comparative example 2;
FIG. 13 is a schematic sectional view showing a stranded wire obtained by
twining wires plated with a Cr--Ni alloy in Example 5 of the present
invention;
FIG. 14(a) is a schematic sectional view showing a flat stranded wire
obtained in Example 6 of the present invention, and FIG. 14(b) is a
perspective view showing a twisted wire for forming the stranded wire;
FIG. 15 is a schematic sectional view showing a secondary stranded wire
obtained by twining 13 primary stranded wires in Example 8;
FIG. 16 is a schematic sectional view showing a stranded wire obtained by
winding wires on a primary stranded wire in Example 9 of the present
invention;
FIG. 17 is a schematic sectional view showing a conductor obtained by
winding stranded wires on a copper pipe in Example 10 of the present
invention; and
FIG. 18 is a schematic sectional view showing a conductor obtained by
winding tape-shaped wires on a copper pipe in two layers in comparative
example 6.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
An oxide superconducting wire according to the present invention is a
multifilamentary wire having such a structure that a plurality of oxide
superconductor filaments are embedded in a stabilizer matrix. In this
structure, each filament is in the form of a ribbon extending along the
longitudinal direction of the wire. Each filament has a rectangular or
substantially rectangular section. The aspect ratio of the ribbon-shaped
filament, i.e., the ratio of the width of the filament to its thickness,
is within the range of 4 to 40, preferably within the range of 4 to 20,
more preferably within the range of 5 to 20. If the aspect ratio is
smaller than 4, it is difficult to sufficiently orient c-axes of crystal
grains for obtaining a superconducting phase exhibiting a high critical
current density. In the wire described in the aforementioned literature
Cryogenics (1992) Vol. 32, No. 11, pp. 940-948, the aspect ratio of each
tape-shaped superconducting filament should be about 3 at the most. It is
presumed that the filament having such a low aspect ratio does not exhibit
a high critical current density due to insufficient orientation of c-axes
of crystal grains. If the aspect ratio of the filament is larger than 40,
on the other hand, it is not easy to form the filament, and longitudinal
bonding of the superconducting phase is extremely easy to separate.
In the wire according to the present invention, the thickness of each
filament is within the range of 5 to 50 .mu.m, preferably within the range
of 10 to 50 .mu.m. If the thickness of the filament is smaller than 5
.mu.m, bonding of the superconducting phase in the longitudinal direction
is extremely easy to separate. If the thickness of the filament is larger
than 50 .mu.m, on the other hand, the ratio of interfacial portions of the
filaments which are in contact with the stabilizing matrix is so small
that it is difficult to sufficiently obtain a superconducting phase having
c-axes oriented in a specific direction. In the filaments having the
aspect ratio of 4 to 40 and the thickness of 5 to 50 .mu.m, c-axes of
crystal grains forming the superconducting phase are oriented
substantially perpendicularly to the longitudinal direction of the wire.
In these ranges, oxide superconducting filaments having sufficient
densities and high critical current densities are attained in the wire
having a circular sectional shape or an at least hexagonal polygonal
sectional shape which is substantially rotation-symmetrical. In relation
to a rotation-symmetrical polygonal sectional shape, that having an angle
of rotation of not more than 90.degree. with respect to a symmetry axis,
i.e., that having an at least four-fold axis of symmetry, is more
preferable.
The present invention provides a wire having a circular or substantially
rotation-symmetrical n-gonal (n: integer of at least 6) section, of which
superconducting properties such as the critical current density and the
like are not substantially varied with the direction of application of a
magnetic field. In the wire according to the present invention, the
plurality of filaments are more preferably arranged at random, or
rotation-symmetrically with respect to the center of the wire, in the
stabilizer matrix. A section of the wire is preferably in the form of a
substantially rotation-symmetrical n-gon (n: even number of at least 6) or
a circle. In the wire having such a section, the aforementioned filaments
bring out a critical current density of at least 2000 A/cm.sup.2.
The structure of the wire according to the present invention is now
described by referring to specific examples. FIGS. 1(a) and 1(b) are
perspective views showing exemplary structures of the wires according to
the present invention respectively, while highlighting sections of the
wires in particular. In an oxide superconducting wire 10 shown in FIG.
1(a), a number of filaments 1 are covered with a stabilizer matrix 3
respectively. The filaments 1 are in the form of ribbons, as shown by
dotted lines. The stabilizer is prepared from silver or silver alloys, for
example. The silver alloys include Ag--Au, Ag--Mn, Ag--Al, Ag--Sb, Ag--Ti
alloys, and the like. A stabilizing matrix 2 having a substantially
regular-hexagonal section is provided at the center of the wire 10 having
a circular section. The filaments 1 covered with the stabilizer are
stacked on each side surface of the hexagon-prismatic stabilizing matrix
2. The filaments 1 are substantially symmetrically arranged with respect
to the center of the wire 10. On the other hand, a wire 11 shown in FIG.
1(b) has a regular-hexagonal section. A hexagon-prismatic stabilizing
matrix 4 is provided at the center of the wire 11, and filaments 1 covered
with a stabilizer 5 are stacked on each side surface thereof. The
filaments 1 are substantially symmetrically arranged with respect to the
center of the wire 11.
FIG. 2 shows still another example having a circular section. A
substantially cylindrical stabilizing matrix 16 is arranged at the center
of an oxide superconducting wire 20, and a number of filaments 15 are
spirally arranged around the matrix 16. A line connecting adjacent ones of
the filaments 15 with each other spirally encloses the matrix 16. The
filaments 15 are in the form of ribbons extending along the longitudinal
direction of the wire 20. The filaments 15 are embedded in a stabilizer
matrix 17.
An oxide superconducting wire 25 shown in FIG. 3 has no large stabilizing
matrix at its center. Alternatively, filaments 21 are arranged throughout
a section of the wire 25. The filaments 21, which are partially layered in
a stabilizer matrix 22, are arranged along various directions, at least in
two directions perpendicular to each other.
The wire according to the present invention is prepared by the so-called
powder-in-tube method. The powder-in-tube method is adapted to obtain a
wire by charging powder of an oxide superconductor or raw material which
can form an oxide superconductor in a tube of a stabilizer and performing
plastic working and a heat treatment on the tube. In preparation of the
raw material powder, powder materials of oxides or carbonates of elements
constituting the superconductor are blended at prescribed blending ratios
and sintered, and the obtained sinter is thereafter pulverized. The tube
which is charged with the powder consists essentially of silver or a
silver alloy, for example. Drawing, rolling, press working or the like is
employed for the plastic working.
In the method of preparing an oxide superconducting wire according to the
present invention, plastic working is performed on the tube which is
charged with the raw material powder, for obtaining a tape-shaped wire.
Drawing and rolling can be employed for obtaining the tape-shaped wire. In
the obtained tape-shaped wire, a portion consisting of the raw material
powder is in the form of a ribbon having an aspect ratio of 4 to 40,
preferably 4 to 20. The tape-shaped wire may be either single-filamentary
or multifilamentary. It is important to set the aspect ratio of the raw
material powder portion in the prescribed range, in order to obtain a wire
having excellent superconducting properties. The obtained tape-shaped wire
is generally cut into a plurality of wires. The obtained plurality of
tape-shaped wires are then charged in a tube consisting essentially of a
stabilizer. The tape-shaped wires are preferably charged in the tube by
methods described later, while the present invention is not restricted to
these method. Plastic working is performed on the tube which is charged
with the tape-shaped wires, to obtain a wire having a substantially
circular or substantially rotation-symmetrical n-gonal (n: integer of at
least 6) section. Drawing can be mainly employed for this plastic working.
A heat treatment is performed on the obtained wire, for forming a sinter
of the oxide superconductor. In the aforementioned process, a wire having
oxide superconducting filaments each having a thickness of 5 to 50 .mu.m
is obtained according to the present invention.
In the step of charging the tape-shaped wires in the tube, a prismatic
stabilizer having a regular n-gonal (n: integer of at least 6) sectional
shape, for example, is provided and the tape-shaped wires can be stacked
around the same. The tape-shaped wires can be stacked around the
stabilizer in one or more layers. The tape-shaped wires are preferably
symmetrically stacked with respect to the center of the stabilizer. In
this charging step, the tape-shaped wires can be arranged in the tube with
substantially no clearance. The process employing this step can produce a
wire having not only a high density of the oxide superconductor but such
an excellent property that superconductivity such as the critical current
density is not much varied with the direction of application of a magnetic
field.
In the step of charging the tape-shaped wires in the tube, alternatively, a
sheet consisting essentially of a stabilizer is first provided and then a
plurality of tape-shaped wires can be arranged thereon in parallel with
each other. Then, the sheet provided with the plurality of tape-shaped
wires is wound on a cylindrical stabilizer. Alternatively, the tape-shaped
wires may be placed around a cylindrical stabilizer together with a
stabilizer sheet for covering and fixing the wire. Through these steps, a
structure having the tape-shaped wires spirally arranged around the
cylindrical stabilizer is obtained. The sheet is spirally wound on the
cylindrical stabilizer. The structure can be inserted in the tube. In the
above steps, the tape-shaped wires can be charged in the tube in a high
density. When plastic working such as drawing is performed on the tube
charged with the tape-shaped wires, force is substantially uniformly
applied to all tape-shaped wires in the tube, whereby uneven distribution
of the superconductors in the section is prevented.
In the step of charging the tape-shaped wires in the tube, further, it is
also preferable to charge the tape-shaped wires in the space of the tube
with substantially no clearance. The charge density of the tape-shaped
wires in the tube is preferably at least 90%. In this case, a plurality of
stacked tape-shaped wires can be first charged in the tube, and then
tape-shaped wires are further charged in the clearances therebetween. In
this step, the tape-shaped wires can be readily charged in the tube in a
high density.
In the step of performing plastic working on the tube charged with the
tape-shaped wires, drawing can be performed. It is more preferable to
employ a driving roller die for the drawing. Referring to FIG. 4, a
driving roller die 28 has two opposite rollers 29a and 29b grooved to
define a specific hole shape. A wire 30 is passed through the hole between
the rollers 29a and 29b, to be drawn. As compared with general hole die
drawing, this drawing has such advantages that (1) the die is less worn,
(2) a larger area reduction ratio can be obtained for a single drawing,
(3) the drawing limit becomes higher, and the like. A wire having a higher
density of the superconducting phase can be obtained by drawing with the
driving roller die. It is more preferable to perform the drawing with a
polygonal driving roller die having rollers which are so grooved that the
drawn wire has a substantially rotation-symmetrical polygonal section. In
this case, the wire is so worked that the superconductor portions therein
become flatter in addition to the aforementioned effects, whereby c-axis
orientation in the superconducting phase of the wire is remarkably
improved.
After the heat treatment is performed on the drawn wire, further drawing
can be performed so that the area reduction ratio of the wire section is
at least 5% and not more than 50%, followed by a further heat treatment.
When the wire is heat treated a plurality of times and further drawing is
performed between the heat treatments, the superconducting phase in the
heat treated wire can be re-arranged so that c-axis orientation is further
increased and the density of the superconducting phase is further
improved. The aforementioned effects cannot be sufficiently attained if
the area reduction ratio is less than 5% in the drawing, while
longitudinal bonding of the superconducting phase is remarkably easy to
separate if the area reduction ratio is in excess of 50%.
The further drawing can be performed with the driving roller die,
preferably the polygonal driving roller die. Due to this step, the effects
of improving the c-axis orientation of the superconducting phase and high
densification thereof can be further increased.
The stabilizer employed in the present invention can be selected from the
group consisting of silver, silver alloys and combinations thereof. The
silver alloys include Ag--Au, Ag--Mn, Ag--Al, Ag--Sb, Ag--Ti alloys, and
the like, but are not restricted to these. When the stabilizer is prepared
from the silver alloy, a wire having high strength and a higher bending or
tensile property can be obtained. According to the present invention, a
wire employing an oxide superconductor such as a bismuth, thallium or
yttrium based oxide superconductor is provided. In particular, each
superconducting filament preferably consists essentially of a bismuth
based oxide superconductor mainly composed of a bismuth 2223 phase such as
Bi.sub.2 Sr.sub.2 Ca.sub.2 Cu.sub.3 O.sub.10-x or (Bi, Pb).sub.2 Sr.sub.2
Ca.sub.2 Cu.sub.3 O.sub.10-x, or a bismuth 2212 phase such as Bi.sub.2
Sr.sub.2 Ca.sub.1 Cu.sub.2 O.sub.8-z or (Bi, Pb).sub.2 Sr.sub.2 Ca.sub.1
Cu.sub.2 O.sub.8-z. In the case of forming a sinter of the bismuth based
oxide superconductor, the heat treatment is preferably performed at a
temperature in the range of 700 to 900.degree. C. As to the bismuth based
oxide superconductor, a superconducting wire which has a high critical
temperature, a high critical current density and low toxicity and can be
readily formed by working is obtained.
A stranded wire according to the present invention is formed by twining a
plurality of the aforementioned oxide superconducting wires, and its
section is flatly shaped. In the twined wires, the aspect ratio of
superconducting filaments is preferably about 10. When the stranded wire
is flatly shaped, the strands are so fully transposed that the impedances
thereof can be equalized to each other in the stranded wire. When a
multifilamentary wire is merely twisted, the filaments may be
insufficiently transposed. The stranded wire having a rectangular section
can be densely wound when applied to a coil or a cable, to advantageously
obtain a compact structure.
The twining step can be carried out after all heat treatment steps as to
wires (round or polygonal wires) having sufficiently high critical
currents. Alternatively, the heat treatment may be performed after the
twining step.
As compared with a twisted multifilamentary wire, transposition is more
desirable in the flatly shaped stranded wire having the twined wires.
While filaments of the twisted wire may be bridged to remarkably reduce
the effect of transposition, such a disadvantage is prevented by such a
stranded wire structure.
Stranded wires prepared according to the present invention can be further
twined. A structure fully transposing all wires can be provided in such an
at least secondary stranded wire, for providing a higher capacitance.
When a high-resistance metal coating layer or an inorganic insulating
coating layer is provided on the outer side the stabilizer of the strands
in the stranded wire, electromagnetic coupling between the twined strands
can be reduced or prevented, the effect of transposition is more
satisfactorily attained, and coupling loss between the strands can be
reduced. The term "high-resistance metal" indicates a metal exhibiting
higher specific resistance than silver employed as the stabilizer. In more
concrete terms, a metal exhibiting resistivity of at least
0.7.times.10.sup.-8 .OMEGA..multidot.m at the liquid nitrogen temperature
of about 77 K and at least 3.times.10.sup.-8 .OMEGA..multidot.m at the
room temperature is preferably employed. The high-resistance metal can
include nickel, chromium or the like.
A method of preparing a stranded wire according to the present invention
can comprise the steps of twining a plurality of wires in which powder of
an oxide superconductor or raw material therefor is covered with a
stabilizer to obtain a stranded wire, flatly shaping the obtained stranded
wire, and heat treating the flatly shaped stranded wire at a temperature
of at least 700.degree. C. and not more than 900.degree. C. In the
process, a plurality of wires in which powder of an oxide superconductor
or raw material therefor is covered with a metal, and which are not yet
finally sintered, are twined into a stranded wire. The number of the
twined wires is preferably 12, 7 or the like, for example. The obtained
stranded wire is flatly shaped as shown in FIG. 5, for example. Thereafter
a heat treatment is performed at a temperature of at least 700.degree. C.
for recovering the stranded wire from deterioration of grain boundaries
resulting from bending in twining or the like, and for completing reaction
if the reaction is insufficient, thereby obtaining a shaped stranded wire
having a high critical current density in which crystal grains of the
oxide superconductor are strongly bonded with each other.
In the step of obtaining the stranded wire, a metal layer having high
resistivity or an inorganic insulating layer can be provided on the outer
side of the stabilizer of silver or a silver alloy. Such a layer can be
prepared by covering a silver pipe with a metal pipe, winding a metal
sheet around the silver pipe, or plating the silver pipe with a metal, for
example. If such a high-resistance metal layer or an inorganic insulating
layer is not provided, silver forming the matrix may be so diffused during
the heat treatment that the wires are disadvantageously bonded with each
other to increase coupling loss between the wires. The high-resistance
layer is effective for reducing such coupling loss. The high-resistance
layer can be formed by an Ag--Au alloy or an Ag--Mn alloy, for example.
Alternatively, Ni or Cr having high resistance may be deposited by
plating. The inorganic insulating layer can be formed by applying a
solution prepared by dispersing powder of an insulator consisting of a
metal oxide such as Al.sub.2 O.sub.3, SiO.sub.2 or the like and then
baking by draying and heating, for example. On the other hand, an oxide
insulating layer may be an MgO layer or a CuO layer prepared by oxidizing
Mg or Cu, for example. Coupling between the wires can be prevented by such
an insulator layer. Further, the effect of transposition is rendered more
desirable. In addition, excellent workability in twining is attained by
depositing Mg or Cu on the wires and then oxidizing Mg or Cu after the
twining.
When the stranded wire is a multifilamentary wire, reduction of the
critical current density can be prevented with respect to bending
distortion in twining. When a twisted multifilamentary wire is employed as
the strand, an effect of transposition of the filaments in the strand is
attained in addition to transposition of the strands.
A conductor having low loss and high capacitance can be obtained by
performing twining and flat-shaping a plurality of times. The obtained
conductor is useful as a compact conductor having low loss and high
capacitance.
Further, a superconducting conductor can be obtained by spirally winding
the stranded wires according to the present invention on a cylindrical
core in one or more layers. The core generally has flexibility. The core,
which is generally called a former, is adapted to hold tape-shaped
superconducting wires in a bending distortion ratio within a prescribed
range. The former has a necessary length for a superconducting cable
conductor, and provided at the center of the superconducting cable
conductor. The former can be provided in a substantially cylindrical or
spiral shape, for winding tape-shaped wires thereon. The former generally
has a substantially constant diameter along its overall length. The former
can consist essentially of at least one material selected from the group
consisting of stainless steel, copper, aluminum and FRP (fiber-reinforced
plastic), for example.
In a single-layer conductor, the positions of all wires can be
electromagnetically equalized by transposition. In this case, current
distribution in the conductor is uniformalized, whereby increase of ac
loss resulting from current drift can be prevented. In case of spirally
winding the wires on the core, it is effective to wind the wires in two
layers along opposite directions, in order to cancel magnetic field
components in the longitudinal direction of the conductor. When the
conductor has at least two layers of the wires, it is desired to prevent
or minimize current drift between the layers resulting from difference
between the impedances of the layers, as well as the increase of ac loss.
EXAMPLE 1
[Preparation of Wire for Engagement]
Bi.sub.2 O.sub.3, PbO, SrCO.sub.3, CaCO.sub.3 and CuO were mixed with each
other so that Bi, Pb, Sr, Ca and Cu were in the ratios of
1.81:0.30:1.92:2.01:3.03. The mixture was repeatedly heat treated and
pulverized, for obtaining powder which was a precursor for an oxide
superconductor. The obtained powder was charged in a silver pipe of 25 mm
in outer diameter and 22 mm in inner diameter, which in turn was drawn to
1.45 mm.O slashed. and then rolled so that its section was 3.2 mm in width
and 0.3 mm in thickness, thereby preparing a tape-shaped wire for
engagement.
[Engagement of Tape-Shaped Wires]
As shown in FIG. 6, wires 36 for engagement obtained in the aforementioned
manner were arranged in five layers along each side surface of a silver
stem 37 which was shaped with a hexagonal die of 3 mm.phi., and engaged in
a silver pipe 38 of 12 mm in outer diameter and 10 mm in inner diameter.
In the arrangement, the tape-shaped wires for engagement obtained through
the aforementioned process were further pressed or rolled, for further
preparing four types of tape-shaped wires which were slightly different in
width from the original ones. The original tape-shaped wires and the newly
obtained four types of tape-shaped wires were arranged around the stem. In
this arrangement, the widths of the tape-shaped wires were outwardly
increased little by little, as shown in FIG. 6. Thus, the wires for
engagement were stably arranged in the silver pipe. Both ends of the pipe
were sealed, and the pipe charged with the tape-shaped wires was
thereafter drawn into 1.63 mm.phi.. The obtained wire is referred to as a
sample a.
As shown in FIG. 7, 35 wires 41 for engagement arranged around a silver
stem 43 of 3 mm.phi. along with a silver sheet 42 were engaged in a silver
pipe 44 of 12 mm in outer diameter and 10 mm in inner diameter. In the
arrangement, all tape-shaped wires 41 were arranged on the silver sheet 42
in parallel with each other and only both ends of each wire were fixed by
an adhesive, and then the silver sheet and the wire were wound on the
stem. Alternatively, the tape-shaped wires may be arranged around the stem
one by one, while covering with and fixing by the silver sheet. The pipe
charged with the tape-shaped wires was drawn into 1.63 mm.phi.. The
obtained wire is referred to as a sample b.
As shown in FIG. 8, wires 47 for engagement were arranged in a silver pipe
48 of 12 mm in outer diameter and 10 mm in inner diameter to be superposed
with each other as many as possible (12 wires in this case), while such
wires 47 for engagement were also arranged in the remaining space in the
silver pipe 48 as many as possible. The pipe charged with the wires was
drawn into 1.63 mm.phi.. The obtained wire is referred to as a sample c.
Similarly to the sample a, wires for engagement were arranged in seven
layers along each side surface of a silver stem, and engaged in a silver
pipe of 12 mm in outer diameter and 10 mm in inner diameter. Then, the
pipe charged with the tape-shaped wires was compressed and drawn with a
polygonal driving roller die 50 shown in FIG. 9. In the polygonal driving
roller die 50, a wire 55 is passed through the hole between driving
rollers 51 and 52 which are rotated through bearings 53 and 54
respectively, for drawing. The wire 55 is worked to have a polygonal
section depending on the hole shape defined by the driving rollers 51 and
52. FIG. 10 shows a section of a wire 60 obtained by drawing with such a
driving roller die. The wire 60 has an octagonal section of about 1.66 mm
in diameter. The obtained wire is referred to as a sample d.
[Heat Treatment]
The samples a to d prepared in the aforementioned manner were subjected to
treatments shown in Table 1 respectively, for preparing oxide
superconducting wires. Referring to Table 1, the obtained wires are
referred to as samples A, B, C, D, A', B', C', D', A", B", C" and D"
respectively.
TABLE 1
Sample
Treatment on Wire a b c d
Heat Treatment 1 .fwdarw. Heat Treatment 2 A B C D
Heat Treatment 1 .fwdarw. Drawing 1 .fwdarw. Heat Treatment 2 A' B' C' D'
Heat Treatment 1 .fwdarw. Drawing 2 .fwdarw. Heat Treatment 2 A" B" C" D"
heat treatment 1: sintering at 845.degree. C. for 50 hours
heat treatment 2: sintering at 840.degree. C. for 50 hours
drawing 1: drawing up to 1.45 mm.phi. for attaining an area reduction ratio
of about 20%
drawing 2: drawing for reducing the diameter to an equivalent diameter of
1.45 mm and attaining an area reduction ratio of about 20% through
compression with the polygonal driving roller die
Comparative Example 1
Wires were prepared as comparative examples as follows:
First, Bi.sub.2 O.sub.3, PbO, SrCO.sub.3, CaCO.sub.3 and CuO were blended
so that Bi, Pb, Sr, Ca and Cu were in the composition ratios of
1.81:0.30:1.92:2.01:3.03. A heat treatment and pulverization were
performed a plurality of times similarly to Example 1, for obtaining
precursor powder. The obtained powder was charged in a silver pipe of 25
mm in outer diameter and 22 mm in inner diameter, which in turn was drawn
to 1.45 mm.phi.. 61 fragments formed by cutting the obtained wire were
bundled and engaged in a silver pipe of 15 mm in outer diameter and 13 mm
in inner diameter, which in turn was drawn to 1.63 mm.phi.. The obtained
wire is referred to as a sample e. The sample e was first subjected to the
aforementioned heat treatment 1, and then subjected to the heat treatment
2 as secondary sintering. The obtained wire is referred to as a sample E.
On the other hand, the sample e was drawn to 1.63 mm.phi., then rolled into
3.6 mm in width and 0.32 mm in thickness, and subjected to the
aforementioned heat treatment 1. Further, the wire was rolled into 3.9 mm
in width and 0.29 mm in thickness and subjected to the aforementioned heat
treatment 2, for obtaining a tape-shaped wire. The obtained wire is
referred to as a sample F.
[Characteristics of Wire]
Samples of 7 cm were formed by the respective wires prepared in Example 1
and comparative example 1, and subjected to measurement of critical
current densities (Jc) at 77 K by a dc four-probe method. Table 2 shows
the results.
TABLE 2
Sample Jc(A/cm.sup.2) Sample Jc(A/cm.sup.2) Sample Jc(A/cm.sup.2)
A 6100 A' 7400 A" 9000
B 5700 B' 6800 B" 8600
C 5100 C' 6000 C" 7500
D 7600 D' 9100 D" 11400
E 1200
F 21000
The samples A to D, A' to D' and A" to D" according to the present
invention exhibited remarkably higher critical current densities Jc than
the comparative round wire (sample E) directly prepared by the
powder-in-tube method, although the values Jc were lower than that of the
tape-shaped wire (sample F). This is conceivably because the tubes charged
with the tape-shaped wires were subjected to drawing. The samples D, D'
and D" drawn by the driving roller die exhibited higher critical current
densities Jc. It is conceivable that the filament portions were further
effectively compacted by the driving roller die.
EXAMPLE 2
[Study of Aspect Ratio of Superconducting Filament in Wire for Engagement]
The wires drawn to 1.45 mm.phi. in the section [preparation of wire for
engagement] in Example 1 were rolled in various working ratios, so that
the internal powder portions were at various aspect ratios. Through the
rolling, wires for engagement were obtained with aspect ratios of 3, 5,
20, 30, 40, 50, 60 and 100 in the powder portions respectively. Similarly
to preparation of the sample, the obtained wires for engagement were
engaged in silver pipes, which in turn were drawn for obtaining round
wires of 1.45 mm.phi.. The heat treatments 1 and 2 were performed to
obtain oxide superconducting wires having circular sections. In the
obtained wires, the aspect ratios of the powder portions were kept
substantially unchanged. In other words, the aspect ratios of the
superconducting filaments were substantially equal to those of the powder
portions. Table 3 shows critical current densities (Jc) of the wires at 77
K with respect to the aspect ratios of the superconducting filaments.
Referring to Table 3, the wire having the aspect ratio of 1 was prepared
by engaging the aforementioned wires which were drawn to 1.45 mm.phi. in a
tube in states of round wires with no rolling.
TABLE 3
Aspect Ratio of
Superconducting
Filament 1 3 5 20 30 40 50 60
100
Jc (77 K. OT) 1200 1800 4100 6400 5400 3900 2000 1400
900
(A/cm.sup.2)
As understood from Table 3, the critical current density Jc is increased
when the aspect ratio of the filaments is increased. If the aspect ratio
exceeds 100, however, the critical current density Jc is reduced to the
contrary. From this experiment, it has been considered preferable to set
the aspect ratio of the superconducting filaments within the range of at
least 4 and not more than 40. A wire having a higher critical current
density Jc is obtained in this range.
[Study of Thickness of Superconducting Filament]
Table 4 shows the relations between the thicknesses of the superconducting
filaments and the critical current densities Jc in the respective wires
shown in Table 3.
TABLE 4
Thickness of
Superconducting
Filament (.mu.m) 3 6 8 10 12 15 40 50
70
Jc (77 K. OT 900 1400 2000 3900 5400 6400 4100 1800
1200
(A/cm.sup.2)
As understood from Table 4, the critical current density Jc is increased as
the thickness of the filaments is reduced. This is because c-axis
orientation of the superconducting phase is promoted in the interface
between the same and the metal portion. If the filaments are further
reduced in thickness, however, the critical current density Jc is reduced.
This is conceivably because longitudinal bonding of the superconducting
phase is separated. From this experiment, it has been considered
preferable to set the thickness of the filaments within the range of 5 to
50 .mu.m.
EXAMPLE 3
[Study of Area Reduction Ratio in Drawing]
Wires of the aforementioned sample a were subjected to the heat treatment
1, and then drawn in various area reduction ratios. Then, the heat
treatment 2 was performed, for obtaining round wires. Table 5 shows the
relations between the area reduction ratios in the drawing and the
critical current densities Jc of the obtained wires. The wire having the
area reduction ratio of 0 was subjected to no drawing.
TABLE 5
Area Reduction
Ratio 0 1 3 50 10 20 37 60
Jc (77 K. OT) 6100 6120 6200 6800 7000 7400 6800 1900
(A/cm.sup.2)
As shown in Table 5, the critical current density Jc of the obtained wire
is improved when drawing is further performed between two sintering steps.
If the area reduction ratio is excessively increased in the drawing,
however, the critical current density Jc is reduced. From this experiment,
it has been understood possible to improve the critical current density of
the wire by drawing the same in an area reduction ratio within the range
of 5 to 50%.
[Preparation of Stranded Wire]
EXAMPLE 4
12 wires of the sample D" prepared in Example 1 were twined and the
obtained stranded wire was flatly shaped so that its section was 8 mm by
2.7 mm. This stranded wire exhibited a critical current density (Ic) of
240 A. FIG. 11 typically shows a section of the obtained flat stranded
wire. 12 wires 60 having substantially rotation-symmetrical octagonal
sections are twined into a flat stranded wire 62.
Comparative Example 2
61 wires of 1.45 mm.phi. prepared in comparative example 1 were bundled and
engaged in a silver pipe of 15 mm in outer diameter and 13 mm in inner
diameter, which in turn was drawn to 1.02 mm.phi.. Then, the obtained wire
was rolled to 0.25 mm, and subjected to the aforementioned heat treatment
1 (at 845.degree. C. for 50 hours). 12 obtained wires were stacked in
layer and rolled to 2.5 mm as to the thickness direction of the stacked
wires and subjected to the aforementioned heat treatment 2 (at 840.degree.
C. for 50 hours), for obtaining a composite wire. FIG. 12 shows the 12
stacked wires 61. Principal surfaces of the wires 61 are superposed with
each other. This composite wire exhibited a critical current density Ic of
320 A.
EXAMPLE 5
Surfaces of wires of the sample D" prepared in Example 1 were subjected to
plating of a Cr--Ni alloy. 12 plated wires were twined and the obtained
stranded wire was flatly shaped so that its section was 8 mm by 2.7 mm.
FIG. 13 shows a section of the obtained stranded wire 65. In the stranded
wire 65, surfaces of wires 66 are provided with plating layers 67 of the
Cr--Ni alloy. Two layers of six transversely arranged plated wires 68 are
superposed with each other. Namely, 12 wires 68 are twined, and the
stranded wire 65 is flatly shaped. This stranded wire exhibited a critical
current density Ic of 380 A.
[Effect of Twining and Plated Layer With Respect to ac Loss]
The stranded wire and the composite wire prepared in Example 4 and
comparative example 2, ac loss values were measured by an energization
four-probe method. When energized under conditions of 60 Hz and peaks of
100 A, the ac loss generated in the stranded wire of Example 4 was 0.6
mW/m, while the wire of comparative example 2 exhibited ac loss of 10
mW/m. It has been understood that ac loss was reduced in the stranded wire
prepared in Example 4. The stranded wire prepared in Example 5 was also
subjected to measurement of ac loss, to prove that the ac loss was further
reduced to 0.12 mW/m when energized under conditions of 60 Hz and a peak
of 100 A. In the following Examples and comparative examples, all ac loss
values were measured by the energization four-probe method.
EXAMPLE 6
In the process of preparing the sample D' in Example 1, the wire was
twisted at pitches of 25 mm before the heat treatment 2. Six twisted wires
were then twined, and the obtained stranded wire was flatly shaped so that
its section was 4.2 mm by 2.8 mm, and subjected to the heat treatment 2.
The obtained stranded wire exhibited a critical current density Ic of 220
A. FIG. 14(a) shows the obtained stranded wire. Six wires 60' having
substantially rotation-symmetrical. octagonal sections are twined into a
flat stranded wire 65'. Each wire 60' is twisted as shown in FIG. 14(b).
Comparative Example 3
In the process of comparative example 2, 61 wires were engaged in a silver
pipe of 15 mm in outer diameter and 13 mm in inner diameter, which in turn
was drawn to 1.45 mm.phi.. Then, the silver pipe was rolled to 0.3 mm, and
subjected to the aforementioned heat treatment 1. Six obtained wires were
stacked in layer, rolled to 3.0 mm, and then subjected to the
aforementioned heat treatment 2. The obtained composite wire exhibited a
critical current density Ic of 250 A.
[Effect of Twisting Before Secondary Sintering With Respect to ac Loss]
The stranded wire and the composite wire prepared in Example 6 and
comparative example 3 were subjected to measurement of ac loss. When
energized under conditions of 60 Hz and peaks of 100 A, the stranded wire
of Example 6 and the wire of comparative example 3 exhibited ac loss
values of 0.7 mW/m and 8 mW/m respectively. It has been understood that
the ac loss is remarkably reduced when the wires to be twined are twisted
before secondary sintering.
EXAMPLE 7
In the process of preparing the sample D' in Example 1, two types of wires
were prepared by plating the wires with Mg and Cu in thicknesses of 10
.mu.m respectively before the heat treatment 2. Then six wires were twined
as to each type, and the obtained stranded wires were flatly shaped and
subjected to the heat treatment 2 similarly to Example 6, for preparing
two types of stranded wires. The Cu and Mg plating layers formed on the
surfaces of the wires were converted to CuO and MgO layers respectively,
so that the wires were substantially completely insulated from each other.
These stranded wires exhibited critical current densities Ic of 350 A. The
Mg and Cu plating layers provided on the surfaces of the wires were so
thin that only oxide films of CuO and MgO were formed on the surfaces of
the heat-treated wires and the superconducting properties of the stranded
wires were not influenced by the plated Mg and Cu.
[Effect of Oxide Film Formed on Wire With Respect to ac Loss]
The two types of stranded wires prepared in Example 7 were subjected to
measurement of ac loss values. The stranded wires formed by the wires with
the CuO and MgO films exhibited ac loss values of 0.1 mW/m and 0.09 mW/m
respectively when energized under conditions of 60 Hz and peaks of 100 A.
It has been confirmed that ac loss, which is coupling loss between the
wires, is remarkably reduced when the wires are covered with oxide films.
EXAMPLE 8
The sample d of 1.15 mm in diameter having an octagonal section prepared in
Example 1 was subjected to the heat treatment 1 and further drawn with a
polygonal driving roller die, for obtaining a wire of 1.02 mm in diameter
having an octagonal section. The obtained wire was subjected to the heat
treatment 2. Four heat treated wires were twined into a primary stranded
wire, and such primary stranded wires were further twined into a secondary
stranded wire. FIG. 15 is a sectional view of the obtained secondary
stranded wire. The secondary stranded wire 70 is formed by twining 13
primary stranded wires 72, each formed by twining four wires 71. Namely,
52 wires 71 are twined in this secondary stranded wire 70.
The obtained secondary stranded wire was further flatly shaped so that its
section was 10.5 mm by 3.5 mm. The obtained secondary stranded wire
exhibited a critical current density Ic of 600 A.
Comparative Example 4
12 wires subjected to the heat treatment 1 in comparative example 3 were
stacked in layer (see FIG. 11), and integrated with each other through the
heat treatment 2. The obtained composite wire exhibited a critical current
density Ic of 620 A.
[Effect of Multiple Stranded Wire With Respect to ac Loss]
The stranded wire and the composite wire prepared in Example 8 and
comparative example 4 were subjected to measurement of ac loss values. The
stranded wire of Example 8 and the composite wire of comparative example 4
exhibited ac loss values of 0.25 mW/m and 3 mW/m respectively when
energized under conditions of 51 Hz and peaks of 200 A. It is understood
that ac loss is reduced in the multiple stranded wire.
EXAMPLE 9
20 wires prepared in Example 1 were wound on a core of the flat stranded
wire prepared in Example 4. Then, the obtained structure was shaped to
have a flat section, for preparing a stranded wire. FIG. 16 shows the
obtained stranded wire. A stranded wire 78 formed by twining 12 wires 76
is provided at the center of the stranded wire 75, and 20 wires 77 are
wound thereon. The stranded wire 75 is flatly shaped so that its section
is 11 mm by 5.2 mm. The obtained stranded wire exhibited a critical
current density Ic of 500 A.
Comparative Example 5
18 wires of comparative example 2 were stacked in layer, rolled to 2 mm and
subjected to the heat treatment 2, for obtaining a composite wire. The
obtained composite wire exhibited a critical current density Ic of 480 A.
[Effect of Flatly Shaped Stranded Wire With Respect to ac Loss]
As a result of measurement of ac loss values, the stranded wire and the
composite wire prepared in Example 9 and comparative example 5 exhibited
ac loss values of 0.3 mW/m and 2 mW/m respectively when energized under
conditions of 50 Hz and peaks of 100 A. It is understood that the ac loss
is reduced in the stranded wire obtained according to the present
invention.
[Preparation of Superconducting Conductor]
EXAMPLE 10
10 flat stranded wire prepared in Example 4 were spirally wound on a copper
pipe of 28 mm in outer diameter, for preparing a superconducting
conductor. FIG. 17 is a sectional view showing the obtained
superconducting conductor. Referring to FIG. 17, 10 stranded wire 82, each
formed by twining 12 wires 81, are spirally wound on a copper pipe 83 in
the conductor 80. The obtained conductor exhibited a critical current
density Ic of 2200 A.
Comparative Example 6
The round wire e prepared in comparative example 1 was rolled so that its
section was 0.46 mm by 5.2 mm, subjected to the heat treatment 1, further
rolled so that its section was 0.41 mm by 5.5 mm, and subjected to the
heat treatment 2, for obtaining a wire. 31 obtained wires were spirally
wound in two layers on a copper pipe of 28 mm in outer diameter, for
preparing a two-layer conductor. FIG. 18 is a sectional view showing the
two-layer conductor. Referring to FIG. 18, wires 88 are spirally wound on
a copper pipe 89 in two layers consisting of inner and outer layers of 15
and 16 wires 88 respectively in the two-layer conductor 90. In the
obtained conductor, each wire exhibited a critical current density Ic of
70 A. The obtained conductor exhibited a critical current density Ic of
2100 A.
[Effectiveness of Single-Layer Stranded Wire Conductor]
When the ac loss values of the conductors prepared in Example 10 and
comparative example 6 were compared with each other, the former was
smaller by two orders than the latter. Thus, effectiveness of the
single-layer stranded wire conductor according to the present invention
was confirmed.
According to the present invention, as hereinabove described, the critical
current density can be improved in an oxide superconducting wire having a
circular sectional shape or an at least hexagonal polygonal sectional
shape which is substantially rotation-symmetrical, by engaging tape-shaped
oxide superconducting wires obtained by pressing or rolling in the pipe
consisting essentially of silver or a silver alloy and drawing the same.
Further, a superconducting stranded wire reducing ac loss can be obtained
by employing the inventive wires. Further, a conductor having small ac
loss can be obtained by employing such stranded wire.
Although the present invention has been described and illustrated in
detail, it is clearly understood that the same is by way of illustration
and example only and is not to be taken by way of limitation, the spirit
and scope of the present invention being limited only by the terms of the
appended claims.
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